The present disclosure relates to semiconductor devices and methods of fabricating semiconductor devices. More particularly, the present disclosure relates to a semiconductor device including a fully silicided (FUSI) gate electrode and a method of fabricating the same.
Generally, a gate electrode of a semiconductor device is formed of polysilicon because a work function can be controlled by implanting dopants into the polysilicon. Thus, a threshold voltage of a transistor may be controlled to be low. For example, in the case of a contemporary metal-oxide semiconductor (CMOS) device where an N-channel MOS (NMOS) transistor and a P-channel MOS (PMOS) transistor are formed parallel to each other, characteristics between the NMOS and PMOS transistors may be readily controlled by forming a gate electrode formed of polysilicon.
However, as semiconductor devices continue to be scaled down, semiconductor devices having a thickness of 50 nanometers or less may encounter difficulties such as depletion and boron penetration which occur at a conventional gate electrode formed of polysilicon. Consequently, a metal gate electrode may be required to address the above-mentioned difficulties.
Unlike with polysilicon, it may be difficult to control a characteristic work function of a metallic material. For this reason, when one material is employed as a gate electrode of a CMOS device, it may be difficult to control characteristics between an NMOS transistor and a PMOS transistor. As a result, two suitable metallic materials may need to be employed to the NMOS transistor and the PMOS transistor, respectively, and thus, a process for forming a CMOS device may become complex and the costs associated therewith may increase.
To overcome the above-mentioned difficulties, methods of forming a fully silicided (FUSI) gate electrode have been proposed in recent years. A FUSI gate electrode is a metal silicide having high melting point formed throughout a gate. In a FUSI gate electrode, an NMOS transistor and/or a PMOS transistor may have a dual work function due to the snowplow effect where dopants injected into polysilicon are segregated at the interface of a gate oxide during an annealing process performed to form a metal silicide. As a result, a CMOS device may be beneficial in readily controlling characteristics between an NMOS transistor and a PMOS transistor and reducing depletion occurring at a gate electrode. Moreover, a silicide is formed at not only a surface of polysilicon but also at the entirety thereof to thereby exhibit improved performance in comparison to a typical metal gate electrode.
Nickel silicide (NiSi) is attractive as a promising silicide material to overcome the difficulties arising from silicide materials such as titanium silicide (TiSi2) or cobalt silicide (CoSi2). Thus, NiSi is increasingly applied to fabrication of high-performance semiconductor devices. Nickel silicide (NiSi) has low resistance and can be silicided at a low temperature, and a small amount of silicon (Si) is consumed to form a silicide having a predetermined thickness. In particular, the amount of silicon in NiSi consumed is significantly smaller than that of silicon in CoSi2. As a result, NiSi is considered to be a silicide material desirable for semiconductor devices having a thin junction.
In view of the foregoing, the study for a FUSI gate electrode acting as a metal gate electrode is focusing on NixSiy. Depending on a deposition thickness of nickel (Ni) and an annealing condition, nickel silicides (NixSiy) may take on various states such as Ni3Si, NiSi, and NiSi2. As a work function varies with the respective states, methods have been developed for forming a state control NixSiy FUSI gate electrode.
The resistivity of NiSi is about 15 to about 20 micro-ohm/cm, which may be suitable for a FUSI gate electrode having low resistance. On the other hand, a work function of NiSi is about 4.6 eV which is equivalent to midgap of silicon used as a semiconductor substrate. Therefore, as a result it may be difficult to apply NiSi to a CMOS device where an NMOS transistor and a PMOS transistor should be used at the same time.
A work function of Ni3Si is about 4.8 eV, which may be applied to a PMOS transistor. A work function of NiSi2 is about 4.4 eV, which may be applied to an NMOS transistor. Nonetheless, as Ni3Si or NiSi2 each have a considerably high resistivity of about 30 to about 100 micro-ohm/cm, it may be difficult to fabricate a FUSI gate electrode having low resistance. As a result, the operating speed and the integration density of a semiconductor device may decrease.